The general art of simulating a radar return signal environment involves the generation of radar specific electromagnetic signals for the purpose of testing the radar system and training the user of the radar system, comprises a complex array of equipment and techniques.
Some equipment arrays are specific to the type of radar because of the unique manner in which the radar transmits its signals. For example, some radars emit periodic pulses of radio frequency energy, some random pulses and others radiate continuously. Their corresponding receivers are linked to the transmitters and only by synchronizing with the transmitter, are they able to discriminate from the many signals received, those which correspond to the signals transmitted and returned from surrounding objects in their field of view. Indeed, even amongst the above categories of radar types, for example, pulse type radars, emitters vary in respect to the width of the pulse, the pulse interval and pulse compression scheme. Some test equipment arrays even replicate the gain patterns of the radar's receiving antenna in an attempt to provide a realistic return signal simulation for testing and mimicing the return environment expected by an operational radar.
One such return signal simulation technique, is to provide a separate oscillator and signal information modulator, for each remote object to be simulated. Each of the signals when suitably modified to represent a return signal are combined and injected at RF to the antenna input of the radar. This approach requires a very large amount of electronic equipment so as to simulate a realistic quantity of remote objects. Each separate oscillator, modulator and their associated RF paths require highly accurate set up. Indeed, some prior return signal generators separately generate each return signal even to the point of using separate, albeit, highly stabilised oscillator circuits to provide long term stability. Regardless of the quality of the circuits and their undoubted expense, the equipment must be calibrated and regularly re-calibrated with, for example, expensive vector network analyser test equipment, operated by skilled calibration engineering staff. Only a high degree of maintenance ensures frequency and phase stability, repeatability and adequate performance during each radar return environment simulation scenario. Without such an approach when using this type of simulation technique, there is a high likelihood that the simulation created will not be representative of real or accurate return signals or the expected radar environment.
Additionally, this technique of return signal generation, although the most obvious to implement is not considered to be economical or maintainable within justifiable budget constraints, particularly if a realistic quantity and quality of remote objects are to be simulated for radar and pilot test purposes.
Another simulation technique is to provide a lesser quantity of separate RF oscillators, and time share them. By rapidly switching a variety of frequency changing and modulation circuits across one RF oscillator a multiple number of simulated remote object radar return signals can be made to appear to be simultaneously generated. These signals are injected at RF level to the radar in place of or combined with its antenna input. Apart from having the same stability problems of the previously described simulation technique, a further disadvantage is that the repetition rate of pulses required to provide a realistic simulation of multiple remote objects is high, which can result in a loss of simulation quality or integrity in dynamic simulation scenarios. This technique cannot simulate a continuous wave return signal nor produce two pulses from two simulated objects at the same time.
A third technique is to simulate signals suitable for insertion into the digital and video section of the radar receiver; however, this technique does not use the RF portion of the radar receiver and thereby tests only the display portion of the radar. Thus this technique restricts the degree of realistic control an operator has to manipulate the total radar system under test.
Apart from simulating a quantity of remote object return signals, the equipment array may also simulate the electromagnetic environment caused by other radar reflectors (RF clutter caused by ground and cloud return signals) and in the case of military aircraft, the use of deceptive and non-deceptive EW counter measure signals.
Various techniques exist for the generation of these types of simulated signals, most significantly it is important, like other simulation signals that these signals should be inserted in a manner that uses a majority of the radar's circuits and provides the most realistic electromagnetic environment in which to test not only the radar operator but also the radar equipment. Particularly in the case of a radar operating during an airborne exercise.
Associated with each simulation technique previously described is the need to control the delay period between a specific radar transmitter pulse and its corresponding return signal since the delay is a direct indication of the distance between the radar transmitter and the remote object returning the signal.
A variety of radar transmission modes can be used by modern pulsed and other radar equipment. For example, the. transmitted pulses may be periodic but they may also be interrupted or the period between pulses can be varied (jittered) as part of the planned operation of the radar. In such circumstances the simulation apparatus must have a mechanism for ordering the delay period for each return signal and its corresponding transmitter pulse. This process is further complicated when the target range is ambiguous, i.e. before the target reflection due to one pulse has been received at the radar, the radar has transmitted one or more extra pulses. (This situation often occurs when the radar uses high or medium pulse repetition frequencies).
In the first instance, for purely periodic transmitter pulse generation it has been common practice to initiate a single counter per return signal generator, to count down the requisite target delay return period, and when the counter times out to initiate the generation of a return signal corresponding to a particular periodically occurring transmitter pulse. In the case where the range is ambiguous, the delay value loaded into the counter is the time between the most recently transmitted radar pulse and the arrival of the target return, rather than the delay corresponding to the time of travel of the pulse which actually caused that target return. This solution, is only useful for periodic modes of transmitter operation because of the way in which ambiguous ranges are handled. Radars which use a jittered mode of signal pulse transmission are not easily simulated at reason able cost, or simplicity of design because of the highly variable nature of the transmit pulse timing, requiring in some techniques multiple counters per target simulation to handle ambiguous near and far field return signal ranges.
Therefore, it is not unusual for radar electromagnetic environment generators to be built at great expense, require constant maintenance, and present unwanted limitations to the quantity of targets used to simulate realistic test and training scenarios. More importantly, they are restricted in application to ground based test equipment because of the bulk and complexity of the equipment needed to provide realistic simulations.
Thus it can be seen that equipment stability, realistic target signals simulation and affordable quantities of target simulation circuits are amongst the problems of providing an acceptable radar electromagnetic environment generator apparatus.